Back pain in a child may not be her back. It could be her kidneys. Take her to the pediatrician for an evaluation. If it's her spine, then she'll need some testing to find out why. Might be a simple muscle strain but you don't take chances with a child.
oz, i totally agree, my sons back hurt him and i put it off but so did he and i finally spoke up and said something in front of the dr. He sent him for exray and and he ended up having spina bifida occulta and something else. Don't wait!